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A 2465 teaser...

Chuck Harris
 

Hi Tom,

It looks just like what you would see if you had a fast
marker pulse, and you sped up the timebase. At slow
timebase settings, it is just a spike, at faster settings,
it is a fast rise, with a much, much slower decay.

The "hooks" are about 5us in duration. The risetime is
very fast, but the decay takes most of the 5us.

I believe that I am looking at two things. Probably issues
with the channel switch:

1) a glitch that is leaking through whenever the sweep starts
drawing CH1/CH2 on the screen.
2) a stream of display glitches that leak through whenever the
channel switch switches between CH1/CH2 and the CH5, the
display.

I have zero doubt that the random looking grassy pulses are
the transitions when the display takes over the beam from CH1/CH2.
They disappear when the display is turned off.

Everything is complicated by the display timing logic. The 2465
tries to avoid the "holes" in the traces that happen when the
display has to happen, so it has 4 or 5 different modes that
range from alternating display and trace, to random display
writing, to writing display full time. The display timing logic
makes these decisions based on how long it has been since it has
refreshed the display.

-Chuck Harris

Tom Gardner tggzzz@... [TekScopes] wrote:

On 10/11/17 06:37, Chuck Harris cfharris@... [TekScopes] wrote:

An additional clue is the leading hook's decay curve changes
slope with sweep speed, but the scope will not trigger on
this signal, nor on any of the random grass that appears on
the trace. That indicates that it is getting injected after
the trigger pickoff.
By that do you mean that when you change the timebase
the decay curve is the same time or the same number
of divisions?

I am pretty certain it has something to do with the display
readout logic.
That was the feeling I got from reading your description.

To see whether it is correlated with the display readout
timing, I would try either of two tests:
1) apply an external signal and adjust its frequency to be
the same as (a harmonic or subharmonic) the display
readout frequency so that any "twinkling" in the traces
is more-or-less stationary. (Start with a timebase of
~200us/div). Then see if the hooks are stationary w.r.t.
the intensity variations. If they are, then the display
readout is involved.
2) without an external signal and a ~2ms/div timebase,
change the holdoff until the twinkling is stationary,
and look for the hooks as above.

If display readout is involved, I would find out what is
happening when the display is switching from trace
to readout. I would observe the Y waveform and blanking
signals after the display sequencer IC and near the
display blanking IC. Trigger on either the blanking
waveform or the Y-waveform.

I would be looking for either the blanking signal to
be mistimed or the Y-waveform to be changing too slowly.
I would suspect the latter, so then it is a case of moving
"away from the output" until you find where it isn't
changing too slowly.

Be careful with probe tips around there; I know they
can short signals and destroy ICs :(

Chuck Harris
 

U400 was high on my list. Its pins have all been given a drop
of deOxit, a second U400 was tried, and the suspect U400 was
tried in the working scope.

I checked the bias resistor, when the hybrid was out, and it measured
correctly, it had continuity to its supply. I checked the HF response
5K pot, it is good, and was adjusted to near full value... which means
the scope would have been a good candidate for the 1K -> 4.3K series
resistor change that happened at later serial numbers.

But, I agree, it would be worth looking at supply and bias connections
a little more carefully.. probably doing something like drawing current
from them to smoke out bad solder joints, etc... that might not be seen
with a low current ohmmeter.

-Chuck Harris

very_fuzzy_logic@... [TekScopes] wrote:

A fault on Ch1, Ch2 but not Ch3, Ch4 or vice versa makes U400 the obvious culprit but you have checked most possibilities. It might be a long shot but incorrect bias current into U400 would probably change its switching speeds and it's worth checking the bias resistor. Also clean the contacts in U400's socket?

Roger

Roger Evans
 

A fault on Ch1, Ch2 but not Ch3, Ch4 or vice versa makes U400 the obvious culprit but you have checked most possibilities. It might be a long shot but incorrect bias current into U400 would probably change its switching speeds and it's worth checking the bias resistor. Also clean the contacts in U400's socket?

Roger

Tom Gardner
 

On 10/11/17 06:37, Chuck Harris cfharris@... [TekScopes] wrote:

An additional clue is the leading hook's decay curve changes
slope with sweep speed, but the scope will not trigger on
this signal, nor on any of the random grass that appears on
the trace. That indicates that it is getting injected after
the trigger pickoff.
By that do you mean that when you change the timebase
the decay curve is the same time or the same number
of divisions?

I am pretty certain it has something to do with the display
readout logic.
That was the feeling I got from reading your description.

To see whether it is correlated with the display readout
timing, I would try either of two tests:
1) apply an external signal and adjust its frequency to be
the same as (a harmonic or subharmonic) the display
readout frequency so that any "twinkling" in the traces
is more-or-less stationary. (Start with a timebase of
~200us/div). Then see if the hooks are stationary w.r.t.
the intensity variations. If they are, then the display
readout is involved.
2) without an external signal and a ~2ms/div timebase,
change the holdoff until the twinkling is stationary,
and look for the hooks as above.

If display readout is involved, I would find out what is
happening when the display is switching from trace
to readout. I would observe the Y waveform and blanking
signals after the display sequencer IC and near the
display blanking IC. Trigger on either the blanking
waveform or the Y-waveform.

I would be looking for either the blanking signal to
be mistimed or the Y-waveform to be changing too slowly.
I would suspect the latter, so then it is a case of moving
"away from the output" until you find where it isn't
changing too slowly.

Be careful with probe tips around there; I know they
can short signals and destroy ICs :(


-Chuck Harris

edbreya@... [TekScopes] wrote:
You may want to check all readily accessible hardware mounting and
grounding, particularly around the delay line. If the hooks are constant, then
they must be leaking into the circuit past the front-end, and somehow
localized so some parts are not affected. The one thing that is commonest to
all sections and operations is the grounding. Check that all screws are
tightened up snug (and that none are missing), and jiggle things around to see
if the symptoms can be aggravated.

To see if you've got some kind of ground loops at the front, jump the
apparently unaffected CH3 and CH4 inputs to the CH1 and CH2 - it's easy to
just use BNC cables for this.

David Az
 

Have you checked the A6 board
Of the front panel
From experience I when i fix my 2465
Failure in one of the diodes can cause such a problem
You can also do a pot test



Exercise 1



And check that all buttons work correctly.



also check the continuity
from J652 on A6 to

( a sec dev /b sec dev)

col 0
col 1
col 2
col 3
col 4
( volts dev ch1 ch2 )

row 0
row 1
row 2
row 3
row 4

Chuck Harris
 

When I first got the scope, it was good, but as I recall,
the fuzz just slowly progressed until it was there always.

That seemed like a capacitor problem, so I started there.

I removed the A1 board to do the capacitor replacement,
and I put all of the screws back myself. They are properly
tightened. That act alone should have refreshed all of the
ground contacts.

The delay line on the 2465 is soldered onto the back of
the A1 board. It doesn't use the little socket pins like
the 465/475 family does.

There is no sensitivity to prodding or smacking the scope
around.

Putting a jumper between CH3/4 and CH1/2 had no effect.

An additional clue is the leading hook's decay curve changes
slope with sweep speed, but the scope will not trigger on
this signal, nor on any of the random grass that appears on
the trace. That indicates that it is getting injected after
the trigger pickoff.

I am pretty certain it has something to do with the display
readout logic.

-Chuck Harris

edbreya@... [TekScopes] wrote:

You may want to check all readily accessible hardware mounting and grounding, particularly around the delay line. If the hooks are constant, then they must be leaking into the circuit past the front-end, and somehow localized so some parts are not affected. The one thing that is commonest to all sections and operations is the grounding. Check that all screws are tightened up snug (and that none are missing), and jiggle things around to see if the symptoms can be aggravated.

To see if you've got some kind of ground loops at the front, jump the apparently unaffected CH3 and CH4 inputs to the CH1 and CH2 - it's easy to just use BNC cables for this.

Ed

Ed Breya
 

You may want to check all readily accessible hardware mounting and grounding, particularly around the delay line. If the hooks are constant, then they must be leaking into the circuit past the front-end, and somehow localized so some parts are not affected. The one thing that is commonest to all sections and operations is the grounding. Check that all screws are tightened up snug (and that none are missing), and jiggle things around to see if the symptoms can be aggravated.

To see if you've got some kind of ground loops at the front, jump the apparently unaffected CH3 and CH4 inputs to the CH1 and CH2 - it's easy to just use BNC cables for this.

Ed

Chuck Harris
 

A hook is a trace that goes from vertical to horizontal
with a little curve between. If you could see the whole
thing, it would look like a shark's tooth, or a lazy
saw tooth pulse.

With display off, it looks like this:

(_____________________________________________CH1

(_____________________________________________CH2

_____________________________________________CH3
_____________________________________________CH4

When you turn display on it looks like:


(__(____(____(______(________(_______(((___(__CH1

(_(__(_____(________(___(_____(_((_____(_(((__CH2

_____________________________________________CH3
_____________________________________________CH4

The "(" sections are actually a sharp rise with
a slower fall, and look like random grass. They
appear only on CH1 and CH2, not at all on CH3 and CH4.

The "(" sections after the trigger are at least
0.75 div high, and unaffected by vertical input
condition, or attenuator setting. The initial pulse
hook, is undoubtedly just one of the "herd"... only
it appears with or without the display being turned
on.

I haven't tried replacing the CH1 and CH2 preamp
yet. It would surprise me to find that both failed
in exactly the same way at the exactly the same time.

[Note that CH1 and CH2 are otherwise performing normally,
and in calibration, and that the "(" stuff does not
appear in the CH2 output connector on the back panel.
I would think that would rule out the preamps.]

It would seem more likely to me that it would be
a bad bypass cap on a filter section that feeds both
CH1 and CH2 in parallel. But the power looks pretty
clean, and nothing larger than 1 or 2 mv.

-Chuck Harris

hahi@... [TekScopes] wrote:

I am puzzled. Anyone have any ideas?
Not sure exactly what you mean by hooks ...

This may not be the same but I have seen a few scopes with noise
on the trace(s) looking like about one minor div of digital noise
randomly across the trace(s). All fixed by replacing the preamp(s).

/Håkan





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Posted by: hahi@...
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I am puzzled. Anyone have any ideas?
Not sure exactly what you mean by hooks ...

This may not be the same but I have seen a few scopes with noise
on the trace(s) looking like about one minor div of digital noise
randomly across the trace(s). All fixed by replacing the preamp(s).

/Håkan

Chuck Harris
 

Hi Gang,

I have a 2465 that spends most of its time on my shelf
as a "should fix this soon" hanger queen.

The problem is thus far, I can't figure out what's wrong
with it.

If you are showing simple ground traces, CH1 and CH2 both
have an identical 0.2 x 0.2 division upward facing "hook" on
the left end of the trace. It moves with the vertical position
control, but is completely unaffected by settings of the
attenuator, and does *not* appear on the CH2 BNC on the rear
panel of the scope... nor would I expect it to, preamps no
nothing of the sweep.

CH3 and CH4 show no sign of the hook. They are perfect.

If you turn on the readout, the hook remains, but there are
3/4 division whispy little hook traces dancing randomly along
the CH1 or CH2 trace. Lots of them. Nothing on the CH3 or CH4
traces. Horizontal rate affects how they are displayed, in the
way you would expect.

My first thought was something that CH1 and CH2 have in common,
the CH3 and CH4 do not.

CH1 and CH2 share some filtered power supply voltages, and have
some power supply voltages that CH3/CH4 do not.

CH1 and CH2 share a couple of opamps in the trigger pickoff
circuitry.

CH1 and CH2 pass through the channel switch through different
channels than CH3 and CH4, and the display readout.

The power supply has been recapped, and tested in another 2465,
that doesn't show the problem.

The Channel switch has been swapped with a 2465 that doesn't
show the problem. All the circuits connections and components
around the channel switch were tested for continuity, shorts,
value...

The Display Sequencer has been swapped with a 2465 that doesn't
show the problem... not that I thought it would make any difference.

The Z axis controller has been swapped with a 2465 that doesn't
show the problem... not that I thought it would make any difference.

All electrolytics on the A1 board have been replaced. What can I
say, the scope is gorgeous, I thought someone might appreciate it.

The noise doesn't appear on the power supplies, or the power rails
at the various hybrids.

All of the published waveforms in the vertical section match the
test points (except for a couple which are known to be wrong in
the manual).

All of the resistors and pots around the channel switch have been
tested. I even pulled U475 and replaced it (because I got a
strange junction measurement that turned out to be ok.

I am puzzled. Anyone have any ideas?

-Chuck Harris